1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine, which oxidizes and reduces noxious components such as Nox, CO and the like in exhaust gas by a catalyst connected to an exhaust system in an engine body.
2. Description of the Prior Art
There is a conventionally known exhaust emission control device, as shown in FIG. 3. Such a prior art device uses, in combination, a main catalyst activated with heat from exhaust gas, and an auxiliary or sub catalyst activated with heat supplied by an electric current.
In such an exhaust emission control device, a self-exothermic type auxiliary catalyst 07 activated with heat supplied by an electric current, a main catalyst 06 activated with the heat from exhaust gas and a silencer 05 are connected in sequence from an upstream side to a downstream side in an exhaust system Ex' leading from an exhaust port in an engine body 01.
In order to activate, at an early stage, the conventional main catalyst which is to be activated with heat from exhaust gas, it is necessary to dispose the catalyst in the vicinity of the engine body, so that it may be exposed to the heat or hot exhaust gas. On the other hand, the self-exothermic type catalyst activated by electric current energy supplied from the outside is capable of being maintained at an activating temperature without relying on the hot exhaust gas. Therefore, if the self-exothermic type catalyst is disposed on the upstream side in the exhaust system in the vicinity of the engine body, as in the above prior art device, it is subjected to the heat of the hot exhaust gas and thus early thermal deterioration to lose its intended function as a catalyst whereby, as a result, it acts as a mere heater.